1. Field of the Invention
The invention relates to a height adjusting device, more particularly to an automatic height adjusting device which is retractable by fluid pressure and expandable by spring action and has provisions to prevent untimely expansion thereof which might otherwise result from the expanding force of a spring member.
2. Description of the Related Art
FIG. 1 is an illustration of a height adjusting device 1 by the applicant, which device 1 is retractable by fluid pressure and expandable by spring action. Stoppers, 12 and 13, plug both ends of an outer cylinder 11 and an inner cylinder 14 disposed inside the outer cylinder 11. A clearance 15 is left between the outer and inner cylinders, 11 and 14. A helical spring (16A) is disposed in the clearance 15 around the inner cylinder 14. A piston 17 is provided inside a fluid reservoir 141 confined by the inner cylinder 14. A piston shaft 171 is connected to the piston 17 and extends through the stopper 13. A slidable piston 18 is disposed in the clearance 15 between the stopper 12 and the helical spring (16A). The slidable piston 18 and the stopper 12 cooperatively confine a fluid receiving space 181. The stopper 12 has a longitudinal shaft opening 122 and a pair of fluid passages 121 which communicate the fluid receiving space 181 with the fluid reservoir 141 confined by the inner cylinder 14 via the shaft opening 122. A shaft 19 has a lowermost end extending through the shaft opening 122 and into the fluid reservoir 141. A valve piece 191 is provided on the lowermost end of the shaft 19 to selectively block the shaft opening 122. The uppermost end of the shaft 19 extends through a recess 123 formed on the topmost surface of the stopper 12. A switching member (A) is provided on the uppermost end of the shaft 19. A helical spring (16B) is disposed on the recess 123 and surrounds an uppermost portion of the shaft 19. One end of the helical spring (16B) is attached to the switching member (A). The helical spring (16B) biases the valve piece 191 to tightly block the shaft opening 122, preventing fluid from flowing between the fluid reservoir 141 and the fluid receiving space 181.
When incorporating the height adjusting device 1 in a height adjustable chair, the lowermost end of the piston shaft 171 is fixed to the chair support stand (not shown). A dust protective cover (B) is provided around the lowermost portion of the outer cylinder 11 to shield the piston shaft 171. The switch member (A) should be conveniently located and is operable by means of a lever or any similar device (not shown).
FIG. 2 is an illustration of the height adjusting device 1 when in use. To adjust the height of a chair incorporating the device 1, the switch member (A) is actuated to move downward, thereby compressing the helical spring (16B) and moving the shaft 19 downward to correspondingly remove the valve piece 191 from the shaft opening 122. The topmost end of the stopper 12 supports a load-bearing article, such as a chair seat (not shown). When downward pressure is applied to the stopper 12, the outer and inner cylinders, 11 and 14, simultaneously move downward. Since the valve piece 191 is displaced from the shaft opening 122, hydraulic fluid inside the fluid reservoir 141 is forced by the piston 17 to flow through the shaft opening 122 and the fluid passages 121 and into fluid receiving space 181. As the volume of hydraulic fluid inside the fluid receiving space 181 gradually increases, the piston 18 consequently moves downward, thereby compressing the helical spring (16A).
When the switch member (A) is released, the helical spring (16B) expands, thereby moving the valve piece 191 to once more block the shaft opening 122. Further downward movement of the outer and inner cylinders, 11 and 14, is stopped, thereby maintaining the chair seat at the desired height.
When the applied downward pressure on the stopper 12 is less than the biasing force of the helical spring (16A), and the valve piece 191 is in an open position, the helical spring (16A) gradually expands to return the stopper 12 to its former unadjusted position.
The foregoing shows that the helical spring (16B) provides the necessary biasing force to block fluid flow, while the helical spring (16A) provides the necessary biasing force to return the stopper 12 to its former unadjusted position.
Although the height adjusting device 1 is an improvement over other conventional types of height adjusting devices, it still has several defects: Referring once more to FIG. 2, the spring modulus of the helical spring (16A) should be relatively large to provide a biasing force strong enough to return the stopper 12 to its unadjusted position. To facilitate operation of the switch member (A), the spring modulus of the helical spring (16B) should be less than that of the helical spring (16A). The valve piece 191 is biased by the helical spring (16B) to block the shaft opening 122. However, the helical spring (16A) urges the piston 18 towards the stopper 12 with a much stronger force when the helical spring (16A) is in a compressed state. Slight upward movement of the piston 18 causes hydraulic fluid in the fluid receiving space 181 to flow into the fluid passages 121 and the shaft opening 122. Fluid in the shaft opening 122 opposes the biasing force of the helical spring (16B) and can remove the valve piece 191 from the shaft opening 122 even when the operator does not intend for this to happen.
A solution to this defect is to make the spring modulus of the helical spring (16B) larger than that of the helical spring (16A). This solution, however, is impractical since the size and space occupied by the helical spring (16B) would be larger than that of the helical spring (16A).